Congestive heart failure (CHF) is a clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet the metabolic demand of peripheral tissues. CHF can be due to a variety of etiologies with that due to ischemic heart disease being the most common. Symptoms of CHF in certain instances can be due to cardiac arrhythmias that are treatable with conventional bradycardia pacing. Some CHF patients suffer from some degree of AV block such that their cardiac output can be improved by synchronizing atrial and ventricular contractions with dual-chamber pacing (i.e., pacing both the atria and the ventricles) using a short programmed AV delay time. It has also been shown that some CHF patients suffer from intraventricular conduction defects (a.k.a. bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of right and left ventricular contractions. Cardiac pacemakers have therefore been developed which provide pacing to both ventricles, termed biventricular pacing.
In CHF patients who are treated with pacing therapy (e.g., either a conventional pacemaker or a biventricular pacemaker), it is desirable to select a pacing scheme that optimally improves the patient's condition. Examples of a pacing scheme include a particular pacing mode and parameter values related to that mode such as lower rate limit, AV delay time, and biventricular delay time. Pacing schemes are conventionally selected based upon a clinical assessment of the patient's condition. For example, EKG data may indicate a patient is likely to be benefited more with biventricular pacing than with conventional dual-chamber pacing. After being set initially, the pacing scheme can then be adjusted on a trial and error basis to a more optimum one based upon the patient's history and physical examination in subsequent office visits. This also allows the pacing scheme to be adjusted in accordance with any changes that occur in the patient's physical condition. This method of assessing a CHF patient's cardiac functional status can be a very subjective one, however, depending on the physician's perception of the patient's symptoms and physical disability. There is a need, therefore, for a method of assessing a CHF patient's functional status that is more accurate and reproducible than those currently practiced in order to select a pacing scheme. It is toward this objective that the present invention is primarily directed.